The title of this poem, leading into the first few lines, surprises us a little. For some reason, we think that "This Is Just To Say" would set up something more significant—like an outright expression of love. Instead, as we find out, it's expressing guilt for eating the plums. But, don't give up that hope for an exclamation of love—this isn't your average note of apology. The care that the speaker takes in writing this note shows, we think, that he cares deeply for the person to whom he's writing the note. Maybe that's why he feels so bad.
you were probably saving for breakfast (6-8)
These lines show that the speaker knew he shouldn't eat the plums, because someone else was saving them, but did it anyway. We can empathize with this—after all, as we see later in the poem, those plums were excellent! And how many times have we stolen a bite from our friend's donut, when they weren't looking. Sometimes, though, doing something we know we're not supposed to be doing gives us guilty pleasure—like eating Take-5s when we're on a diet. At least, in this case, Williams has the sense to leave a note apologizing for his guilty pleasure.
Forgive me they were delicious so sweet and so cold (9-12)
Okay, so clearly this guy knows he did wrong, but what's the deal here? He's issuing a command—Forgive me, not please forgive me—and then he goes on to describe how awesome these plums are. We're thinking his wife might say, yeah hon, I know, that's why I was saving them, you jerk. But we have to remember that we're dealing with not just a note of apology here, but also a poem. And someone who would write a poem to apologize might consider that including some delicious imagery of a delicious moment would remove some of the sting. If they can't actually eat the plums, at least they can enjoy a poem about eating the plums.